Added support for conditional banners via pattern

I was looking for a way to only include banner for a subset of my sources, which was not supported. 

I've implemented a new option called "pattern", which is tested against the file contents. The banner will only be injected if there is a match. The option is default off, so will not affect existing usage. Tests included.


This is very neat if you are working with IE typescript, and need to add /* istanbul ignore next */ to all those pesky "var __extends = this.__extends || function (d, b) {" statements. Hope others can benefit!